Method of manufacturing thin film, substrate having thin film, electron emission material, method of manufacturing electron emission material, and electron emission device

ABSTRACT

A method of manufacturing a thin film, including: mixing carbon nanofibers into an elastomer including an unsaturated bond or a group having affinity to the carbon nanofibers, and dispersing the carbon nanofibers by applying a shear force to obtain a carbon fiber composite material; mixing the carbon fiber composite material and a solvent to obtain a coating liquid; and applying the coating liquid to a substrate to form a thin film.

This is a divisional of application Ser. No. 11/282,614, filed Nov. 21,2005 and claims the benefit of Japanese Patent Application No.2004-337620, filed on Nov. 22, 2004, and Japanese Patent Application No.2005-307394, filed on Oct. 21, 2005. The entire disclosures of the priorapplications are hereby incorporated by reference in their entirety.

BACKGROUND OF THE INVENTION

The present invention relates to a method of manufacturing a thin film,a substrate having a thin film, an electron emission material and amethod of manufacturing the same, and an electron emission device.

In recent years, application of carbon nanofibers to electromagneticdevices has been studied.

For example, a method of manufacturing a thin film by causing the carbonnanofibers to be directly grown on a substrate has been proposed (e.g.JP-A-11-349307).

However, the size and the shape of the thin film are limited when usingthe method of causing the carbon nanofibers to be directly grown on asubstrate. Moreover, the resulting substrate is expensive due to lowmanufacturing efficiency.

A method of manufacturing a thin film by spraying a dispersion liquidcontaining the carbon nanofibers has also been proposed (e.g.JP-A-2003-121892).

However, since the carbon nanofibers are generally produced in the formof an aggregated powder or a bundle, the carbon nanofibers are notuniformly dispersed in the dispersion liquid. Therefore, it is difficultto improve the dispersibility of the carbon nanofibers in the resultingthin film.

In recent years, an electron emission device in which electrons areemitted upon application of an electric field has been proposed for adisplay (field emission display: FED) such as a thin television or aflat lighting device in order to deal with a demand for energy saving.The electron emission device is required to allow electron emission at alow electric field and have a high current density and long life. Thecarbon nanofibers proposed as an electron emission material for theelectron emission device can achieve a high current density at a lowelectric field. However, it was found that the carbon nanofibers breakduring electron emission and therefore have a short life (e.g.JP-A-2003-77386).

SUMMARY

According to a first aspect of the invention, there is provided a methodof manufacturing a thin film, the method comprising:

mixing carbon nanofibers into an elastomer including an unsaturated bondor a group having affinity to the carbon nanofibers, and dispersing thecarbon nanofibers by applying a shear force to obtain a carbon fibercomposite material;

mixing the carbon fiber composite material and a solvent to obtain acoating liquid; and

applying the coating liquid to a substrate to form a thin film.

According to a second aspect of the invention, there is provided asubstrate having a thin film obtained by the above-described method.

According to a third aspect of the invention, there is provided anelectron emission material comprising a thin film obtained by theabove-described method.

According to a fourth aspect of the invention, there is provided anelectron emission material comprising a carbon fiber composite materialincluding an elastomer and carbon nanofibers dispersed in the elastomer.

According to a fifth aspect of the invention, there is provided a methodof manufacturing an electron emission material, the method comprising:

mixing carbon nanofibers into an elastomer including an unsaturated bondor a group having affinity to the carbon nanofibers, and dispersing thecarbon nanofibers by applying a shear force to obtain a carbon fibercomposite material.

According to a sixth aspect of the invention, there is provided anelectron emission device comprising:

a cathode including any of the above-described electron emissionmaterials; and

an anode disposed at a specific interval from the cathode,

electrons being emitted from the electron emission material by applyingvoltage between the anode and the cathode.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

FIG. 1 is a view schematically showing a mixing method for an elastomerand carbon nanofibers utilizing an open-roll method according to oneembodiment of the invention.

FIG. 2 is a view schematically showing solution application by using aspin coater according to one embodiment of the invention.

FIG. 3 is a view schematically showing aggregated carbon nanofibers andelectrical conduction.

FIG. 4 is a view schematically showing the state of carbon nanofibersand electrical conduction in a thin film according to one embodiment ofthe invention.

FIG. 5 is a schematic enlarged view showing a part of a carbon fibercomposite material (electron emission material) according to oneembodiment of the invention.

FIG. 6 is a schematic view showing a configuration of a field emissiondisplay using an electron emission device according to one embodiment ofthe invention.

FIG. 7 is a schematic view showing a configuration of a flat lightingdevice according to one embodiment of the invention.

FIG. 8 is a schematic view showing a configuration of a flat lightingdevice according to one embodiment of the invention.

FIG. 9 is a schematic view showing a configuration of a flat lightingdevice according to one embodiment of the invention.

FIG. 10 is a schematic view showing a configuration of a flat lightingdevice according to one embodiment of the invention.

FIG. 11 is a schematic view showing a configuration of a curved lightingdevice according to one embodiment of the invention.

FIG. 12 is a schematic view showing a configuration of a tubularlighting device according to one embodiment of the invention.

DETAILED DESCRIPTION OF THE EMBODIMENT

The invention may provide a method of manufacturing a thin film in whichcarbon nanofibers are uniformly dispersed, a substrate having the thinfilm, an electron emission material and a method of manufacturing thesame, and an electron emission device.

According to one embodiment of the invention, there is provided a methodof manufacturing a thin film, the method comprising:

mixing carbon nanofibers into an elastomer including an unsaturated bondor a group having affinity to the carbon nanofibers, and dispersing thecarbon nanofibers by applying a shear force to obtain a carbon fibercomposite material;

mixing the carbon fiber composite material and a solvent to obtain acoating liquid; and

applying the coating liquid to a substrate to form a thin film.

A thin film in which the carbon nanofibers are uniformly dispersed isformed on the substrate obtained by this method of manufacturing a thinfilm. In the step of obtaining the carbon fiber composite material, theunsaturated bond or group of the elastomer bonds to an active site ofthe carbon nanofiber, particularly to a terminal radical of the carbonnanofiber, to reduce the aggregating force of the carbon nanofibers, sothat the carbon nanofibers can be uniformly dispersed in the elastomeras a matrix. A coating liquid in which carbon nanofibers are suspendedis obtained by dissolving the carbon fiber composite material, in whichthe carbon nanofibers are uniformly dispersed, in a solvent. This isbecause the carbon nanofibers are uniformly suspended in the coatingliquid without precipitating in the solvent due to high wettabilitybetween the carbon nanofibers and the elastomer. A thin film in whichthe carbon nanofibers are uniformly dispersed can be formed on thesubstrate by applying the coating liquid to the substrate.

The thin film in this embodiment may have a g-value of a signal of anunpaired electron of carbon measured at 4.5° K by using an electron spinresonance spectrometer of 2.000 or more and less than 2.002. Since theg-value of a metal is 2.000, the thin film in this embodiment having ag-value within such a range has an electrical conductivity similar tothat of a metal. Moreover, the thin film may have a line width of asignal of an unpaired electron of carbon measured at 4.5° K by using anelectron spin resonance spectrometer of 300 μT or more. Such a linewidth indicates that the carbon nanofibers are uniformly dispersed, sothat the thin film in this embodiment has an electrical conductivitysimilar to that of a metal.

The elastomer used in this embodiment may be a rubber elastomer or athermoplastic elastomer. When using a rubber elastomer, the elastomermay be in a crosslinked form or an uncrosslinked form. However, a rubberelastomer in an uncrosslinked form is preferable since the carbonnanofibers are easily mixed. A network component of the elastomer in anuncrosslinked form may have a spin-spin relaxation time (T2 n) measuredat 30° C. by a Hahn-echo method using a pulsed nuclear magneticresonance (NMR) technique of 100 to 3,000 μsec. A network component ofthe elastomer in an crosslinked form may have a spin-spin relaxationtime (T2 n) measured at 30° C. by a Hahn-echo method using a pulsednuclear magnetic resonance (NMR) technique of 100 to 2,000 μsec.

In this method of manufacturing a thin film, the step of dispersing thecarbon nanofibers in the elastomer by applying a shear force may beperformed by:

(a) an open-roll method including tight milling with a roll interval of0.5 mm or less;

(b) an internal mixing method;

(c) a multi-screw extrusion kneading method; or the like.

According to one embodiment of the invention, there is provided anelectron emission material comprising a thin film obtained by theabove-described method.

According to one embodiment of the invention, there is provided anelectron emission material comprising a carbon fiber composite materialincluding an elastomer and carbon nanofibers dispersed in the elastomer.

The above electron emission materials enable electron emission at a lowelectric field while maintaining long life of the carbon nanofibers byencapsulating the carbon nanofibers with the elastomer. Since theelectron emission materials have an electrical conductivity similar tothat of a metal while using the elastomer as the matrix, the electronemission materials enable electron injection. Moreover, since theelectron emission materials include the elastomer as the matrix, thedegrees of freedom of the form of the electron emission materials arehigh, so that it is possible to flexibly deal with a number ofapplications.

According to one embodiment of the invention, there is provided a methodof manufacturing an electron emission material, the method comprising:

mixing carbon nanofibers into an elastomer including an unsaturated bondor a group having affinity to the carbon nanofibers, and dispersing thecarbon nanofibers by applying a shear force to obtain a carbon fibercomposite material.

In this method of manufacturing an electron emission material, anelectron emission material including a carbon fiber composite materialin which the carbon nanofibers are uniformly dispersed in the elastomeris obtained. According to this manufacturing method, an electronemission material which has an electrical conductivity similar to thatof a metal while using the elastomer as the matrix and allows efficientelectron emission can be obtained.

According to one embodiment of the invention, there is provided anelectron emission device comprising:

a cathode including any of the above-described electron emissionmaterials; and

an anode disposed at a specific interval from the cathode,

electrons being emitted from the electron emission material by applyingvoltage between the anode and the cathode.

This electron emission device can reduce power consumption whilemaintaining long life.

These embodiments of the invention are described below in detail withreference to the drawings.

The elastomer preferably has characteristics such as a certain degree ofmolecular length and flexibility in addition to high affinity to thecarbon nanofibers. In the step of dispersing the carbon nanofibers inthe elastomer by applying a shear force, it is preferable that thecarbon nanofibers and the elastomer be mixed at as high a shear force aspossible.

(I) Elastomer

The elastomer has a molecular weight of preferably 5,000 to 5,000,000,and still more preferably 20,000 to 3,000,000. If the molecular weightof the elastomer is within this range, since the elastomer molecules areentangled and linked, the elastomer easily enters the space between theaggregated carbon nanofibers to exhibit an improved effect of separatingthe carbon nanofibers. If the molecular weight of the elastomer is lessthan 5,000, since the elastomer molecules cannot be entangledsufficiently, the effect of dispersing the carbon nanofibers is reducedeven if a shear force is applied in the subsequent step. If themolecular weight of the elastomer is greater than 5,000,000, since theelastomer becomes too hard, processing becomes difficult.

The network component of the elastomer in an uncrosslinked form has aspin-spin relaxation time (T2 n/30° C.), measured at 30° C. by aHahn-echo method using a pulsed nuclear magnetic resonance (NMR)technique, of preferably 100 to 3,000 μsec, and still more preferably200 to 1,000 μsec. If the elastomer has a spin-spin relaxation time (T2n/30° C.) within the above range, the elastomer is flexible and has asufficiently high molecular mobility. Therefore, when mixing theelastomer and the carbon nanofibers, the elastomer can easily enter thespace between the carbon nanofibers due to high molecular mobility. Ifthe spin-spin relaxation time (T2 n/30° C.) is shorter than 100 μsec,the elastomer cannot have a sufficient molecular mobility. If thespin-spin relaxation time (T2 n/30° C.) is longer than 3,000 μsec, sincethe elastomer tends to flow as a liquid, it becomes difficult todisperse the carbon nanofibers.

The network component of the elastomer in a crosslinked form preferablyhas a spin-spin relaxation time (T2 n), measured at 30° C. by theHahn-echo method using the pulsed NMR technique, of 100 to 2,000 μsec.The reasons therefor are the same as those described for theuncrosslinked form. Specifically, when crosslinking an uncrosslinkedform which satisfies the above conditions by using the production methodof the invention, the spin-spin relaxation time (T2 n) of the resultingcrosslinked form almost falls within the above range.

The spin-spin relaxation time obtained by the Hahn-echo method using thepulsed NMR technique is a measure which indicates the molecular mobilityof a substance. In more detail, when measuring the spin-spin relaxationtime of the elastomer by the Hahn-echo method using the pulsed NMRtechnique, a first component having a shorter first spin-spin relaxationtime (T2 n) and a second component having a longer second spin-spinrelaxation time (T2 nn) are detected. The first component corresponds tothe network component (backbone molecule) of the polymer, and the secondcomponent corresponds to the non-network component (branched componentsuch as terminal chain) of the polymer. The shorter the first spin-spinrelaxation time, the lower the molecular mobility and the harder theelastomer. The longer the first spin-spin relaxation time, the higherthe molecular mobility and the softer the elastomer.

As the measurement method in the pulsed NMR technique, a solid-echomethod, a Carr-Purcell-Meiboom-Gill (CPMG) method, or a 90-degree pulsemethod may be applied instead of the Hahn-echo method. However, sincethe carbon fiber composite material according to the invention has amedium spin-spin relaxation time (T2), the Hahn-echo method is mostsuitable. In general, the solid-echo method and the 90-degree pulsemethod are suitable for measuring a short spin-spin relaxation time(T2), the Hahn-echo method is suitable for measuring a medium spin-spinrelaxation time (T2), and the CPMG method is suitable for measuring along spin-spin relaxation time (T2).

At least one of the main chain, side chain, and terminal chain of theelastomer includes an unsaturated bond or a group having affinity to thecarbon nanofiber, particularly to a terminal radical of the carbonnanofiber, or the elastomer has properties of readily producing such aradical or group. The unsaturated bond or group may be at least oneunsaturated bond or group selected from a double bond, a triple bond,and functional groups such as α-hydrogen, a carbonyl group, a carboxylgroup, a hydroxyl group, an amino group, a nitrile group, a ketonegroup, an amide group, an epoxy group, an ester group, a vinyl group, ahalogen group, a urethane group, a biuret group, an allophanate group,and a urea group.

The carbon nanofiber generally has a structure in which the side surfaceis formed of a six-membered ring of carbon atoms and the end is closedby introduction of a five-membered ring. However, since the carbonnanofiber has a forced structure, a defect tends to occur, so that aradical or a functional group tends to be formed at the defect. In oneembodiment of the invention, since at least one of the main chain, sidechain, and terminal chain of the elastomer includes an unsaturated bondor a group having high affinity (reactivity or polarity) to the radicalof the carbon nanofiber, the elastomer and the carbon nanofiber can bebonded. This enables the carbon nanofibers to be easily dispersed byovercoming the aggregating force of the carbon nanofibers.

As the elastomer, an elastomer such as natural rubber (NR), epoxidizednatural rubber (ENR), styrene-butadiene rubber (SBR), nitrile rubber(NBR), chloroprene rubber (CR), ethylene propylene rubber (EPR or EPDM),butyl rubber (IIR), chlorobutyl rubber (CIIR), acrylic rubber (ACM),silicone rubber (Q), fluorine rubber (FKM), butadiene rubber (BR),epoxidized butadiene rubber (EBR), epichlorohydrin rubber (CO or CEO),urethane rubber (U), or polysulfide rubber (T); a thermoplasticelastomer such as an olefin-based elastomer (TPO), poly(vinylchloride)-based elastomer (TPVC), polyester-based elastomer (TPEE),polyurethane-based elastomer (TPU), polyamide-based elastomer (TPEA), orstyrene-based elastomer (SBS); or a mixture of these elastomers may beused. The inventors of the invention confirmed that it is particularlydifficult to disperse the carbon nanofibers in ethylene propylene rubber(EPR, EPDM).

(II) Carbon Nanofiber

It is preferable that the carbon nanofibers have an average diameter of0.5 to 500 nm. The carbon nanofiber may be either a linear fiber or acurved fiber.

The amount of carbon nanofibers added is not particularly limited, andmay be determined depending on the application. For example, in order toensure that the g-value of a signal of an unpaired electron of carbonmeasured at 4.5° K by using an electron spin resonance (ESR)spectrometer is 2.000 or more and less than 2.002 as described later, itis preferable that the carbon nanofiber content in the carbon fibercomposite material be 10 to 40 vol % when using multi-wall carbonnanofibers. When using single-wall carbon nanofibers, it is preferablethat the carbon nanofiber content in the carbon fiber composite materialbe 0.2 to 40 vol % in order to ensure that the g-value of a signal of anunpaired electron of carbon at 4.5° K is 2.000 or more and less than2.002. In the carbon fiber composite material according to oneembodiment of the invention, a crosslinked elastomer, an uncrosslinkedelastomer, or a thermoplastic polymer may be directly used as theelastomer material.

As examples of the carbon nanofiber, a carbon nanotube and the like canbe given. The carbon nanotube has a single-layer structure in which agraphene sheet of a hexagonal carbon layer is closed in the shape of acylinder, or a multi-layer structure in which the cylindrical structuresare nested. Specifically, the carbon nanotube may be formed only of thesingle-layer structure or the multi-layer structure, or the single-layerstructure and the multi-layer structure may be present in combination. Acarbon material having a partial carbon nanotube structure may also beused. The carbon nanotube may be called a graphite fibril nanotube.

A single-layer carbon nanotube or a multi-layer carbon nanotube isproduced to a desired size by using an arc discharge method, a laserablation method, a vapor-phase growth method, or the like.

In the arc discharge method, an arc is discharged between electrodematerials made of carbon rods in an argon or hydrogen atmosphere at apressure slightly lower than atmospheric pressure to obtain amulti-layer carbon nanotube deposited on the cathode. When a catalystsuch as nickel/cobalt is mixed into the carbon rod and an arc isdischarged, a single-layer carbon nanotube is obtained from sootadhering to the inner side surface of a processing vessel.

In the laser ablation method, a target carbon surface into which acatalyst such as nickel/cobalt is mixed is irradiated with strong pulselaser light from a YAG laser in a noble gas (e.g. argon) to melt andvaporize the carbon surface to obtain a single-layer carbon nanotube.

In the vapor-phase growth method, a carbon nanotube is synthesized bythermally decomposing hydrocarbons such as benzene or toluene in a vaporphase. As specific examples of the vapor-phase growth method, a floatingcatalyst method, a zeolite-supported catalyst method, and the like canbe given.

The carbon nanofibers may be provided with improved adhesion to andwettability with the elastomer by subjecting the carbon nanofibers to asurface treatment such as an ion-injection treatment, sputter-etchingtreatment, or plasma treatment before mixing the carbon nanofibers intothe elastomer.

As the carbon nanofibers used as the electron emission material,single-wall carbon nanotubes (SWNTs), double-wall carbon nanotubes(DWNTs), and multi-wall carbon nanotubes (MWNTs) having an averagediameter of less than 100 nm are preferable. In particular, DWNTsexhibit excellent electron emission properties. The carbon nanofibersused as the electron emission material preferably have an average lengthof about 20 μm. The content (filling rate) of the carbon nanofibers inthe carbon fiber composite material is preferably 0.1 to 40 vol %.

(III) Mixing Carbon Nanofibers into Elastomer and Dispersing CarbonNanofibers by Applying Shear Force to Obtain Carbon Fiber CompositeMaterial

In one embodiment of the invention, an example using an open-roll methodincluding tight milling with a roll interval of 0.5 mm or less isdescribed below as the step of mixing the metal particles and the carbonnanofibers into the elastomer.

FIG. 1 is a diagram schematically showing the open-roll method using tworolls. In FIG. 1, a reference numeral 10 indicates a first roll, and areference numeral 20 indicates a second roll. The first roll 10 and thesecond roll 20 are disposed at a predetermined interval d (e.g. 1.5 mm).The first and second rolls are rotated normally or reversely. In theexample shown in FIG. 1, the first roll 10 and the second roll 20 arerotated in the directions indicated by the arrows.

When an elastomer 30 is caused to be wound around the second roll 20while rotating the first and second rolls 10 and 20, a bank 32 of theelastomer is formed between the rolls 10 and 20. After the addition ofcarbon nanofibers 40 to the bank 32, the first and second rolls 10 and20 are rotated to obtain a mixture of the elastomer and the carbonnanofibers. The mixture is then removed from the open rolls. Aftersetting the interval d between the first roll 10 and the second roll 20at preferably 0.5 mm or less, and still more preferably 0.1 to 0.5 mm,the mixture of the elastomer and the carbon nanofibers is supplied tothe open rolls and tight-milled. Tight milling is preferably performedabout ten times, for example. When the surface velocity of the firstroll 10 is indicated by V1 and the surface velocity of the second roll20 is indicated by V2, the surface velocity ratio (V1/V2) of the firstroll 10 to the second roll 20 during tight milling is preferably 1.05 to3.00, and still more preferably 1.05 to 1.2. A desired shear force canbe obtained by using such a surface velocity ratio.

This causes a high shear force to be applied to the elastomer 30 so thatthe aggregated carbon nanofibers are separated in such a manner that thecarbon nanofibers are removed by the elastomer molecules one by one andare dispersed in the elastomer 30.

When metal or nonmetal particles are supplied to the bank 32 beforesupplying the carbon nanofibers, a shear force produced by the rollscauses turbulent flows to occur around the metal or nonmetal particlesso that the carbon nanofibers can be further dispersed in the elastomer30.

In this step, the elastomer and the carbon nanofibers are mixed at arelatively low temperature of preferably 0 to 50° C., and still morepreferably 5 to 30° C. in order to obtain as high a shear force aspossible. When using EPDM as the elastomer, it is preferable to performtwo-stage mixing steps. In the first mixing step, EPDM and the carbonnanofibers are mixed at a first temperature which is 50 to 100° C. lowerthan the temperature in the second mixing step in order to obtain ashigh a shear force as possible. The first temperature is preferably 0 to50° C., and still more preferably 5 to 30° C. A second temperature ofthe rolls is set at a relatively high temperature of 50 to 150° C. sothat the dispersibility of the carbon nanofibers can be improved.

Since the elastomer according to one embodiment of the invention has theabove-described characteristics, specifically, the above-describedmolecular configuration (molecular length), molecular motion, andchemical interaction with the carbon nanofibers, dispersion of thecarbon nanofibers is facilitated. Therefore, a carbon fiber compositematerial exhibiting excellent dispersibility and dispersion stability(carbon nanofibers rarely reaggregate) can be obtained. In more detail,when mixing the elastomer and the carbon nanofibers, the elastomerhaving an appropriately long molecular length and a high molecularmobility enters the space between the carbon nanofibers, and a specificportion of the elastomer bonds to a highly active site of the carbonnanofiber through chemical interaction. When a high shear force isapplied to the mixture of the elastomer and the carbon nanofibers inthis state, the carbon nanofibers move accompanying the movement of theelastomer, whereby the aggregated carbon nanofibers are separated anddispersed in the elastomer. The dispersed carbon nanofibers areprevented from reaggregating due to chemical interaction with theelastomer, whereby excellent dispersion stability can be obtained.

In the step of dispersing the carbon nanofibers in the elastomer byapplying a shear force, the above-described internal mixing method ormulti-screw extrusion kneading method may be used instead of theopen-roll method. In other words, it suffices that a shear forcesufficient to separate the aggregated carbon nanofibers be applied tothe elastomer.

A carbon fiber composite material obtained by the step of mixing anddispersing the carbon nanofibers in the elastomer (mixing and dispersionstep) may be formed after crosslinking the carbon fiber compositematerial by using a crosslinking agent, or may be formed withoutcrosslinking the carbon fiber composite material.

In the mixing and dispersing step of the elastomer and the carbonnanofibers, or in the subsequent step, a compounding, ingredient usuallyused in the processing of an elastomer such as rubber may be added. Asthe compounding ingredient, a known compounding ingredient may be used.As examples of the compounding ingredient, a crosslinking agent,vulcanizing agent, vulcanization accelerator, vulcanization retarder,softener, plasticizer, curing agent, reinforcing agent, filler, agingpreventive, colorant, and the like can be given.

FIG. 5 is a cross-sectional view schematically showing the carbon fibercomposite material according to one embodiment of the invention. In acarbon fiber composite material 1 according to one embodiment of theinvention obtained by the above-described step, the carbon nanofibers 40are uniformly dispersed in the elastomer 30 as a matrix. An interfacialphase 36 is formed around the carbon nanofiber 40. The interfacial phase36 is considered to be an aggregate of the molecules of the elastomer 30formed when the molecular chain of the elastomer 30 is cut duringmixing, and free radicals produced attack and adhere to the surface ofthe carbon nanofiber 40. The interfacial phase 36 is considered to besimilar to bound rubber formed around carbon black when mixing anelastomer and carbon black, for example. The interfacial phase 36 coversand protects the carbon nanofiber 40. Moreover, the elastomer is dividedby the chain of the interfacial phases 36 so that small cells 34 of theelastomer having a nanometer size and enclosed by the interfacial phases36 are formed. The interfacial phase 36 prevents breakage of the carbonnanofiber 40 due to electron emission by covering the carbon nanofiber40, so that an electron emission material having improved life can beobtained. The carbon fiber composite material according to oneembodiment of the invention may be used as an electron emission materialin the form of a thin film described later, or may be used as anelectron emission material in another form depending on the application.For example, the carbon fiber composite material may be used in the formof a sheet obtained by using the open roll method. Or, the carbon fibercomposite material obtained by the above-described step may be formedinto a complicated shape by using an injection molding method, atransfer molding method, a press molding method, or the like, or may beformed into a product having a continuous shape, such as a sheet shape,an angular cylindrical shape, or a round cylindrical shape, by using anextrusion method, a calendering method, or the like. The elastomer inthe carbon fiber composite material may be either crosslinked oruncrosslinked.

The carbon nanofibers are generally entangled and dispersed in a mediumto only a small extent. However, since the carbon nanofibers aredispersed in the elastomer in the carbon fiber composite materialaccording to one embodiment of the invention, the carbon nanofibers canbe easily dispersed in a medium by dissolving the carbon fiber compositematerial as a raw material in a solvent, for example.

(IV) Mixing Carbon Fiber Composite Material and Solvent to ObtainCoating Liquid

The step of obtaining a coating liquid according to one embodiment ofthe invention includes mixing the carbon fiber composite material and asolvent. The carbon fiber composite material according to one embodimentof the invention does not precipitate when dissolved in a solvent due toexcellent wettability between the carbon nanofibers and the elastomer.This is because the carbon nanofibers are uniformly suspended in thecoating liquid in a state in which the carbon nanofibers are entangledwith the dissolved elastomer molecules. Moreover, the carbon nanofibersexist in the coating liquid while being covered with the interfacialphase.

As the solvent used in this step, a solvent including at least one ofaromatic hydrocarbon solvents such as toluene and xylene and alicyclichydrocarbon solvents such as cyclohexane may be appropriately selecteddepending on the type of elastomer. The solvent may be appropriatelyselected corresponding to the elastomer from organic solvents such astoluene, benzene, cyclohexane, thinner (mixed solvent), ethylene glycol,monoethyl ether (cellosolve), ethylene glycol monoethyl ether acetate(cellosolve acetate), ethylene glycol monobutyl ether (butylcellosolve), ethylene glycol monomethyl ether (methyl cellosolve),ortho-dichlorobenzene, chlorobenzene, chloroform, carbon tetrachloride,1.4-dioxane, 1.2-dichloroethane (ethylene dichloride).1,2-dichloroethylene (acetylene dichloride), 1,1,2,2-tetrachloroethane(acetylene tetrachloride), xylene, N,N-dimethylformamide, styrene,tetrachloroethylene (perchloroetyhlene), trichloroethylene,1,1,1-trichloroethane, carbon disulfide, n-hexane, acetone, isobutylalcohol, isopropyl alcohol, isopentyl alcohol, ethyl ether, ethyleneglycol monoethyl ether, ortho-dichlorobenzene, xylene (ortho), xylene(meta), xylene (para), cresol (ortho), cresol (meta), cresol (para),isobutyl acetate, isopropyl acetate, isopentyl acetate, ethyl acetate,butyl acetate, propyl acetate, pentyl acetate, methyl acetate,cyclohexanol, 1,4-dioxane, dichloromethane, tetrahydrofuran, n-hexane,1-butanol, 2-butanol, methanol, methyl isobutyl ketone, methyl ethylketone, methylcyclohexanol, methylcyclohexanone, methyl butyl ketone,industrial gasoline, coal tar naphtha (solvent naphtha), petroleumether, petroleum naphtha (light), petroleum naphtha (heavy), petroleumbenzine, turpentine oil, and mineral spirit. For example, toluene isused when the elastomer in the carbon fiber composite material isnatural rubber (NR) or styrene-based elastomer (SBS), and cyclohexane isused when the elastomer is EPDM.

(V) Applying Coating Liquid to Substrate to Form Thin Film

In the step of applying the coating liquid to a substrate to form a thinfilm according to one embodiment of the invention, a method of applyingthe coating liquid to a substrate to a uniform thickness may beemployed. The application method is preferably performed by using amethod selected from a spin coating method, a dipping method, a screenprinting method such as electrostatic painting, a spraying method, andan inkjet method. The applied coating liquid is freeze-dried orthermally dried in a reduced-pressure thermostat or cured by applicationof ultraviolet rays to form a thin film. The thickness of the thin filmis preferably 0.5 to 10 μm, although the thickness of the thin filmdiffers depending on the formation method for the thin film.

In one embodiment of the invention, an example using a spin coatingmethod is described below as the step of applying the coating liquid tothe substrate.

As shown in FIG. 2, a disk-shaped substrate 60 is placed on a substratesupport stage 70 connected with a motor 80, for example. The substrate60 is secured on the substrate support stage 70 by causing the substrate60 to adhere to the substrate support stage 70 under vacuum by usingvacuum means (not shown) provided in the substrate support stage 70, andthe substrate support stage 70 and the substrate 60 are rotated by usingthe motor 80 at a rotational speed of 2000 rpm, for example. A coatingliquid 100 obtained by the above-described (e) is applied over theentire surface of the substrate 60 by dripping the coating liquid 100onto the substrate 60 from an application nozzle 90 while rotating thesubstrate 60. The applied liquid is freeze-dried in a reduced-pressurethermostat to form a thin film on the substrate 60.

As the material for the substrate 60, a metal such as gold, copper, oraluminum, a semiconductor such as a silicon wafer, glass, a polymermaterial, or the like may be used.

Since the carbon nanofibers are uniformly suspended in the coatingliquid obtained by the above-described (d) without precipitating in thecoating liquid, the carbon nanofibers can be uniformly dispersed overthe substrate by using the spin coating method.

(VI) Thin Film Formed on Substrate

The carbon nanofibers are uniformly dispersed in the thin film formed onthe substrate by using the method according to one embodiment of theinvention. The thin film according to one embodiment of the inventionmay be used as an electromagnetic material or an electron emissionmaterial.

The dispersion state of the carbon nanofibers in the thin film may bedetermined by subjecting the thin film to measurement by the Hahn-echomethod using the pulsed NMR technique.

The spin-lattice relaxation time (T1) measured by the Hahn-echo methodusing the pulsed NMR technique is a measure indicating the molecularmobility of a substance together with the spin-spin relaxation time(T2). In more detail, the shorter the spin-lattice relaxation time ofthe thin film, the lower the molecular mobility and the harder the thinfilm. The longer the spin-lattice relaxation time of the thin film, thehigher the molecular mobility and the softer the thin film.

In the thin film, the carbon nanofibers are uniformly dispersed in theelastomer as the matrix. In other words, the elastomer is restrained bythe carbon nanofibers. The mobility of the elastomer moleculesrestrained by the carbon nanofibers is low in comparison with the casewhere the elastomer molecules are not restrained by the carbonnanofibers. Therefore, the first spin-spin relaxation time (T2 n), thesecond spin-spin relaxation time (T2 nn), and the spin-latticerelaxation time (T1) of the thin film according to one embodiment of theinvention are shorter than those of the elastomer which does not containthe carbon nanofibers. The spin-lattice relaxation time (T1) of the thinfilm in a crosslinked form changes in proportion to the amount of carbonnanofibers mixed.

In a state in which the elastomer molecules are restrained by the carbonnanofibers, the number of non-network components (non-reticulate chaincomponents) is considered to be reduced for the following reasons.Specifically, when the molecular mobility of the elastomer is entirelydecreased by the carbon nanofibers, since the number of non-networkcomponents which cannot easily move is increased, the non-networkcomponents tend to behave in the same manner as the network components.Moreover, since the non-network components (terminal chains) easilymove, the non-network components tend to be adsorbed on the active sitesof the carbon nanofibers. It is considered that these phenomena decreasethe number of non-network components. Therefore, the fraction (fnn) ofcomponents having the second spin-spin relaxation time is smaller thanthat of the elastomer which does not contain the carbon nanofibers. Thefraction (fn) of components having the first spin-spin relaxation timeis greater than that of the elastomer which does not contain the carbonnanofibers, since “fn+fnn=1”.

Therefore, the thin film according to one embodiment of the inventionpreferably has values measured by the Hahn-echo method using the pulsedNMR technique within the following range.

Specifically, it is preferable that the thin film in an uncrosslinkedform have a first spin-spin relaxation time (T2 n) measured at 110° C.of 100 to 3,000 pee, a second spin-spin relaxation time (T2 nn) measuredat 110° C. of either 0 μsec or 1,000 to 10,000 μsec, a fraction (fn) ofcomponents having the first spin-spin relaxation time of 0.95 or more,and a fraction (fnn) of components having the second spin-spinrelaxation time of less than 0.05.

The dispersion state of the carbon nanofibers in the thin film accordingto one embodiment of the invention may be determined by subjecting thethin film to line width measurement by using an electron spin resonance(hereinafter called “ESR”) spectrometer. The electromagnetic propertiesof the thin film according to one embodiment of the invention may bedetermined by measuring the g-value of a signal of an unpaired electronof carbon by using the ESR spectrometer.

The ESR spectrometer applies microwaves to an unpaired electron (spin)and allows microwave absorption to be observed as a spectrum.

The g-value measured by using the ESR spectrometer is an apparent indexwhen a free radical having an unpaired electron absorbs microwaves andan energy filed in a magnetic field at a specific intensity. A largerg-value indicates a higher resonance energy absorption. Therefore, theg-value characterizes a free radical. The line width measured by usingthe ESR spectrometer is an index indicating the interaction betweenunpaired electrons. The measurement of the g-value and the line width ofa signal of an unpaired electron of carbon by using the ESR spectrometeris carried out at a temperature of 4.5° K at which a signal of aconduction electron is not detected.

The thin film according to one embodiment of the invention preferablyhas a g-value of a signal of an unpaired electron of carbon measured at4.5 K by using the ESR spectrometer of 2.000 or more and less than2.002. Since the g-value of a metal is 2.000, it is understood that thethin film according to the invention having a g-value within such arange has an electrical conductivity similar to that of a metal.

The electrical conductivity of the thin film is described below withreference to FIGS. 3 and 4. FIGS. 3 and 4 are views schematicallyshowing the state of the carbon nanofibers and electrical conduction inthe thin film.

In general, when the carbon nanofibers aggregate in the elastomer,electrical conduction occurs on the side surface of the carbon nanofiber(arrow 50 in FIG. 3) and inside the carbon nanofiber (arrow 52 in FIG.3), as shown in FIG. 3. In this state, since electrical conductionoccurring on the side surface of the carbon nanofiber (arrow 50 in FIG.3) is predominant, the g-value of a signal of an unpaired electron ofcarbon measured by using the ESR spectrometer is 2.0023.

However, when the carbon nanofibers are uniformly dispersed as in thethin film according to one embodiment of the invention, electricalconduction occurring inside the carbon nanofiber (arrow 52 in FIG. 4) ispredominant, as shown in FIG. 4. Moreover, electrical conduction occursat a location at which the carbon nanofibers are in contact with eachother (arrow 53 in FIG. 4). As a result, the thin film has an electricalconductivity similar to that of a metal (g-value: 2.000).

The thin film according to the invention having a g-value within theabove-mentioned range preferably has a line width of a signal of anunpaired electron of carbon measured at 4.5° K by using the ESRspectrometer of 300 μT or more. Such a line width indicates that thecarbon nanofibers are uniformly dispersed in the thin film according tothe invention and that the thin film has an electrical conductivitysimilar to that of a metal.

It is preferable that the thin film have a high tensile strength. Thethin film according to the invention has a tensile strength higher thanthat of the raw material elastomer, and the tensile strength can beincreased by increasing the carbon nanofiber content.

The thin film according to one embodiment of the invention is ahigh-efficiency electron emission material having a threshold electricfield of 4 V/μm or less and a saturation current density of 10 mA/cm² ormore. The thin film according to one embodiment of the invention enableselectron emission at a low electric field while maintaining the life ofthe carbon nanofiber by encapsulating the carbon nanofiber with theelastomer, particularly the interfacial phase. Since the electronemission material according to one embodiment of the invention has anelectrical conductivity similar to that of a metal while using theelastomer as the matrix, the electron emission material enables electroninjection. Moreover, since the electron emission material includes theelastomer as the matrix, the degrees of freedom of the form of theelectron emission material are high, so that it is possible to flexiblydeal with a number of applications. The elastomer forming the thin filmmay be either crosslinked or uncrosslinked.

(VII) Electron Emission Device

FIG. 6 is a schematic view showing a configuration of a field emissiondisplay (FED) 110 using an electron emission device according to oneembodiment of the invention. The field emission display 110 includes acathode 8, in which a thin film (electron emission material) 2 obtainedby the above-described step is formed on an electrode substrate 60, anda glass substrate 5, which is disposed at a predetermined interval fromthe cathode 2 through a gate electrode 4, in a vacuum airtightcontainer, for example. An anode 6 and a fluorescent body 7 are formedin layers on the glass substrate 5 on the side of the cathode 2.Therefore, the field emission display 110 includes an electron emissiondevice including the cathode 8 including the thin film 2, the anode 6,and the gate electrode 4 disposed between the cathode 8 and the anode 6.

When applying voltage between the cathode 8 and the gate electrode 4,electrons (e⁻) are emitted from the surface of the thin film 2 formed ofthe electron emission material on the side of the gate electrode 4toward the anode 6. The electrons (e⁻) emitted from the cathode 8progress toward the anode 6, and an image can be displayed by utilizingemission of light occurring when the electrons collide with thefluorescent body 7. An emitter as a protrusion electron emission sectionmay be formed by subjecting the surface of the thin film 2 of thecathode 8 to a surface treatment such as etching. Or, since the entiresurface of the cathode 8 is formed by the thin film 2 formed of theelectron emission material, the surface of the cathode 8 can function asan emitter without etching the surface of the cathode 8.

The electron emission device exhibits high electron emission efficiencydue to the carbon nanofibers dispersed in the entire thin film 2, andallows easy electron injection since the thin film 2 has an electricalconductivity equal to that of a metal. Moreover, since the carbonnanofibers are covered with the elastomer, particularly the interfacialphase, the carbon nanofibers have long life.

The electron emission material and the electron emission device thusobtained may be used for various applications in addition to the fieldemission display. For example, a surface emission body (surfacefluorescent body) may be formed by causing emission of light over theentire surface of the electrode substrate, or the electron emissionmaterial and the electron emission device may be used for variouselectrodes utilizing electron emission by a hot cathode operation or acold cathode operation for a fluorescent lamp, an electron microscope, aplasma display, or the like.

FIGS. 7 to 12 are schematic vertical cross-sectional views showingconfigurations of lighting devices using the electron emission materialaccording to one embodiment of the invention.

A flat lighting device 200 shown in FIG. 7 includes a cathode 160 inwhich the carbon fiber composite material (electron emission material)obtained by the above-described step is formed as an electrodesubstrate, a glass plate 120 which is disposed at a predeterminedinterval from the cathode 160 and in which a fluorescent pigment film130 is formed on the side of the cathode 160, a spacer 150 whichdetermines the interval between the glass plate 120 and the cathode 160,and a grid (anode) 140 formed between the glass plate 120 and thecathode 160. The glass plate 120, the grid 140, and the cathode 160 arein the shape of a quadrilateral flat plate, for example. The grid 140 isa metal plate having a plurality of minute openings formed by punching,electroforming, or the like. The glass plate 120 is transparent. Thefluorescent pigment film 130 is applied to the surface of the glassplate 120 on the side of the cathode 160 by using a screen printingmethod or the like. The spacer 150 having a specific thickness isdisposed at the outer edge of the flat glass plate 120 and the cathode160 and is interposed between the glass plate 120 and the cathode 160 sothat an airtight vacuum space 180 is formed between the glass plate 120and the cathode 160. The outer edge of the grid 140 is inserted into andsecured by the middle portion of the spacer 150. When applying voltagebetween the cathode 160 and the grid 140, electrons are emitted from thesurface of the cathode 160 formed of the electron emission material onthe side of the grid 140 toward the glass substrate 120, and passthrough the minute openings in the grid 140. The electrons which havebeen emitted from the cathode 160 and passed through the minute openingsin the grid 140 progress toward the anode 120, and emission of lightoccurs when the electrons collide with the fluorescent pigment film 130.The space between the cathode 160 and the glass plate 120 may beevacuated, or may be filled with a specific gas such as argon gas. Theglass plate 120 may be transparent as in one embodiment of theinvention, or may be colored in the same manner as in a known lightingdevice.

A flat lighting device 202 shown in FIG. 8 is the same as the embodimentshown in FIG. 8 except that a cathode thin film 162 is formed on asubstrate 170 formed of aluminum or the like. The cathode thin film 162is a thin film obtained by thinly applying the carbon fiber compositematerial (electron emission material) obtained by the above-describedstep to the substrate 170.

A flat lighting device 204 shown in FIG. 9 is the same as the embodimentshown in FIG. 7 except that the grid shown in FIG. 7 is removed and theflat lighting device 204 includes a transparent ITO glass plate 122 onwhich an anode 124 is formed on the side of the cathode 160. When usingthe ITO glass plate 122, the fluorescent pigment film 130 is applied tothe anode 124 formed on the surface of the ITO glass plate 122 on theside of the cathode 160 by using a screen printing method or the like.Specifically, the anode 124 is disposed between the ITO glass plate 122and the fluorescent pigment film 130 when using the ITO glass plate 122.Therefore, when applying voltage between the anode 124 and the cathode160, electrons are emitted from the surface of the cathode 160 (electronemission material) toward the ITO glass plate 122 and collide with thefluorescent pigment film 130 so that emission of light occurs. Afluorescent pigment film may be applied to a transparent glass plateinstead of the ITO glass plate 122 by using a screen printing method,and an aluminum thin film anode may be formed on the fluorescent pigmentfilm by using a vacuum deposition method or the like.

A flat lighting device 206 shown in FIG. 10 has a configuration similarto that of the field emission display (FED) 110 shown in FIG. 6, inwhich the grid 140 is added between the ITO glass 122 and the cathode160 shown in FIG. 9. As described above, since the flat lighting devices200 to 206 are thin and emit light at low power consumption, the flatlighting devices 200 to 206 can be installed as a part of a buildinginner wall material.

In a curved lighting device 208 shown in FIG. 11, a part of the ITOglass plate 122, the grid 140, the cathode 162, and the substrate 170forms a curved surface. The shape of the lighting device can bearbitrarily designed by curving the electrode and the like. Therefore,the degrees of freedom of the shape of the lighting device can beincreased when using the lighting device for building applications. Asthe pigment of the fluorescent pigment film 130, a white fluorescentpigment generally used in a lighting device is preferable. However, afluorescent pigment of another color may be selected as required.

A tubular lighting device 210 shown in FIG. 12 is a fluorescent lamptype lighting device having a circular cross-sectional shape, and has aconfiguration essentially the same as that of the curved lighting device208 shown in FIG. 11. The anode 124 is formed over the inner surface ofthe glass enclosure 121 obtained by forming an ITO glass plate in theshape of a tube, and the fluorescent pigment film 130 is formed over theanode 124. Each end of the glass enclosure 121 is sealed with caps 152,and the sealed space 180 is maintained under vacuum. A narrow columnarelectrode 172 is disposed at the center of the tubular lighting device210, and each end of the electrode 172 is secured to the caps 152. Theouter surface of the electrode 172 is covered with the cathode thin film162. The grid 140 disposed in the shape of a tube at a specific intervalfrom the cathode thin film 162 encloses the electrode 172. Therefore,when applying voltage between the grid 140 and the cathode thin film162, electrons are emitted from the surface of the cathode thin film 162formed of the electron emission material radially toward the grid 140.The electrons pass through the minute openings in the grid 140 andcollide with the fluorescent pigment film 130 so that light is emittedfrom the entire tube. The tubular lighting device 210 is a lightingdevice having excellent recyclability, since the lighting device 210 issimilar to a known fluorescent lamp and does not contain mercury insidethe tube.

Examples according to the invention and comparative examples aredescribed below. However, the invention is not limited to the followingexamples.

Examples 1 to 10 and Comparative Examples 1 to 3 (1) Preparation ofSample

(a) Preparation of Carbon Fiber Composite Material

Step 1: A predetermined amount (100 g) of elastomer shown in Tables 1and 2 was supplied to open rolls having a roll diameter of six inches(roll temperature: 10 to 20° C.), and the elastomer was wound around theroll.

Step 2: Carbon nanofibers (indicated as “MWNT” in Table 1 and “SWNT” inTable 2) were added to the elastomer in an amount (vol %) shown inTables 1 and 2. The roll interval was set at 1.5 mm.

Step 3: After the addition of the carbon nanofibers, the mixture of theelastomer and the carbon nanofibers was removed from the rolls.

Step 4: After reducing the roll distance from 1.5 mm to 0.3 mm, themixture was supplied to the rolls and tight-milled. The surface velocityratio of the rolls was set at 1.1. Tight milling was repeatedlyperformed ten times.

Step 5: After setting the roll interval at a predetermined interval (1.1mm), the mixture subjected to tight milling was supplied to the rollsand sheeted.

“MWNT” shown in Tables 1 and 4 indicates multi-wall carbon nanotubeshaving an average diameter of 13 nm (manufactured by ILJIN Nanotech Co.,Ltd.), and “SWNT” shown in Tables 2 and 4 indicates single-wall carbonnanotubes having an average diameter of 1 nm (manufactured by ILJINNanotech Co., Ltd.). “E-SBS” shown in Tables 1 to 4 indicates astyrene-butadiene block copolymer having an epoxy content of 1.7 wt %and a styrene content of 40 wt %.

In Example 4 using EPDM, the roll temperature in the step 4 was set at100° C. and the mixing operation was performed for 20 min in order toimprove the dispersibility of the carbon nanofibers.

Carbon fiber composite materials of Examples 1 to 10 were thus obtained.In Comparative Example 1, a carbon fiber composite material was obtainedby using carbon fibers having an average diameter of 28 μm (“CF” inTable 1) instead of the carbon nanofibers. In Comparative Example 2, acarbon fiber composite material was obtained by using HAF-grade carbonblack having an average particle diameter of 28 nm (“HAF-CB” in Table 1)instead of the carbon nanofibers.

(b) Preparation of Coating Liquid

1 g of the carbon fiber composite material obtained in each of Examples1 to 10 and Comparative Examples 1 and 2 was added to 100 g of a solventand dissolved with stirring to obtain a coating liquid.

As the solvent, toluene was used in Examples 1 to 4, 6, and 7 to 10 andComparative Examples 1 and 2 (elastomer: natural rubber or styrenebutadiene block copolymer), and cyclohexane was used in Example 5(elastomer: EPDM).

In Comparative Example 3, 50 g of the elastomer was dissolved in 100 gof toluene without preparing a carbon fiber composite material. Afterthe addition of MWNT, the mixture was stirred to obtain a coatingliquid.

(c) Preparation of Thin Film

A glass substrate placed on a spin coater was rotated at 2000 rpm. Thecoating liquid obtained by (b) in Examples 1 to 10 and ComparativeExamples 1 to 3 was applied dropwise to the glass substrate to uniformlyspread the coating liquid over the glass substrate.

The coating liquid spread over the substrate was freeze-dried at −70° C.in a reduced-pressure thermostat to form a thin film with a thickness of5 μm on the glass substrate.

The thin film was removed from the glass substrate and subjected to thefollowing measurements (2) to (4).

(2) Measurement Using Pulsed NMR Technique

The thin film obtained in each of Examples 1 to 10 and ComparativeExamples 1 to 3 was subjected to measurement by the Hahn-echo methodusing the pulsed NMR technique. The measurement was conducted using“JMN-MU25” manufactured by JEOL, Ltd. The measurement was conductedunder conditions of an observing nucleus of ¹H, a resonance frequency of25 MHz, and a 90-degree pulse width of 2 μsec, and a decay curve wasdetermined while changing Pi in the pulse sequence (90° x-Pi-180° x) ofthe Hahn-echo method. The sample was measured in a state in which thesample was inserted into a sample tube within an appropriate magneticfield range. The measurement temperature was set at 110° C. in order toprevent thermal deterioration. The first spin-spin relaxation time (T2n) and the fraction (fn) of components having the first spin-spinrelaxation time of the thin film were determined by this measurement.The measurement results are shown in Tables 1 and 2.

The first spin-spin relaxation time (T2 n) and the fraction (fn) ofcomponents having the first spin-spin relaxation time of each elastomerare shown in Table 3.

(3) Measurement of Tensile Properties

A sample with a thickness of 1 mm was prepared by using the thin film ofeach of Examples 1 to 10 and Comparative Examples 1 to 3. The tensilestrength of the sample was measured according to JIS K 6521-1993. Theresults are shown in Tables 1 and 2.

(4) Measurement Using ESR Spectrometer

The g-value and the line width (μT: microtesla) of a signal of anunpaired electron of carbon were measured for the thin films of Examples1 to 10 and Comparative Examples 1 to 3 by using an ESR spectrometer.The measurement was conducted by using “JES-FA” manufactured by JEOL,Ltd. The thin film of each of Examples 1 to 10 and Comparative Examples1 to 3 was cut into a strip-shaped sample with a weight of about 3 mg.The sample was inserted into a sample tube. The measurement wasconducted at a temperature of 4.5° K, a magnetic field sweep of 10 mT(millitesla), and an oscillation frequency of 9 GHz using manganese (Mn)as a standard. The results are shown in Tables 1 and 2.

TABLE 1 Example 1 2 3 4 5 Carbon fiber Elastomer Natural rubber (NR)(vol %) 99.6 98.6 80 70 — composite EPDM (vol %) — — — — 80 materialE-SBS (vol %) — — — — — Carbon fiber MWNT (vol %) 0.4 1.4 20 30 20 CF(vol %) — — — — — HAF-CB (vol %) — — — — — Thin film Thickness (μm) 5 55 5 5 NMR properties T2n 1460 1170 1380 950 695 (383K) fn 0.88 0.956 1 10.98 Tensile properties Tensile strength (MPa) 7.5 7.5 24.7 33.1 17.1Elongation (%) 340 240 130 100 120 ESR properties g-value 2.0023 2.00182.0015 2.0001 2.0016 (4.5K) Line width (μT) 204 354 820 935 850 ExampleComparative Example 6 1 2 3 Carbon fiber Elastomer Natural rubber (NR)(vol %) — 80 80 (80) composite EPDM (vol %) — — — — material E-SBS (vol%) 80 — — — Carbon fiber MWNT (vol %) 20 — — (20) CF (vol %) — 20 — —HAF-CB (vol %) — — 20 — Thin film Thickness (μm) 5 5 5  5 NMR propertiesT2n 1100 1450 1380 2200  (383K) fn 1 0.941 0.916    0.92 Tensileproperties Tensile strength (MPa) 44 1.3 21   0.8 Elongation (%) 250 280195 50 ESR properties g-value 2.0015 2.0023 2.0023     2.0023 (4.5K)Line width (μT) 825 185 191 736 

TABLE 2 Example 7 8 9 10 Carbon fiber Elastomer Natural rubber (NR) (vol%) 99.6 98.6 96 92 composite EPDM (vol %) — — — — material E-SBS (vol %)— — — — Carbon fiber SWNT (vol %) 0.4 1.4 4 8 CF (vol %) — — — — HAF-CB(vol %) — — — — Thin film Thickness (μm) 5 5 5 5 NMR properties T2n 14501420 1400 1370 (383K) fn 0.98 1 1 1 Tensile properties Tensile strength(MPa) 21.6 18.9 16.4 13.3 Elongation (%) 450 280 210 165 ESR propertiesg-value 2.0016 2.0013 2.0011 2.0004 (4.5K) Line width (μT) 450 560 610892

TABLE 3 NMR properties (383K) Elastomer T2n fn Natural rubber (NR) 14500.86 EPDM 615 0.88 E-SBS 1740 0.869

As is clear from Tables 1 to 3, the following items were confirmedaccording to Examples 1 to 10 of the invention. Specifically, thespin-spin relaxation time at 110° C. (T2 n/110° C.) of the thin filmincluding the carbon nanofibers was shorter than those of the elastomerand the thin film of Comparative Example 3. The thin film including thecarbon nanofibers had a fraction (fn/110° C.) greater than those of theelastomer and the thin film of Comparative Example 3. These resultssuggest that the carbon nanofibers were uniformly dispersed in the thinfilm according to the example.

As is clear from the results of the tensile properties of the thin film,it was confirmed that the thin films according to Examples 1 to 10 ofthe invention had an improved tensile strength due to inclusion ofuniformly dispersed carbon nanofibers to exhibit a reinforcing effect.This is more clearly understood by comparing Examples 1 to 10 withComparative Example 3 in which the carbon nanofibers were insufficientlydispersed.

It was confirmed from the thin films of Examples 2 to 4 using MWNT thatthe g-value of a signal of an unpaired electron of carbon in the ESRproperties approaches 2.000, which is the g-value of a metal, as thecarbon nanofiber content is increased. Since the thin film of Example 1had a low. MWNT content (0.4 vol %), the thin film of Example 1 had ag-value of 2.0023. The above-described tendency was confirmed for thethin films of Examples 7 to 10 using SWNT at an SWNT content lower thanthe MWNT content. Specifically, while the thin film containing MWNT hada g-value of 2.0001 at an MWNT content of 30 vol %, the thin film ofExample 10 containing SWNT had a g-value of 2.0004 at an SWNT content aslow as 8 vol %. It was confirmed that the g-value of the thin film alsoapproaches 2.000 (g-value of metal) when using EPDM or thestyrene-butadiene block copolymer as in Examples 5 and 6. The thin filmof Comparative Example 3, in which the carbon nanofibers wereinsufficiently dispersed, had a g-value of 2.0023.

The thin films of Examples 2 to 10 had a line width of a signal of anunpaired electron of carbon of 300 μT or more. This indicates that thecarbon nanofibers were uniformly dispersed and the spin (unpairedelectron) concentration was high. Since the line width of a signal of anunpaired electron of carbon in the ESR properties may show a similartendency for an unpaired electron in the carbon nanofiber aggregate asin Comparative Example 3, the dispersibility of the carbon nanofiberswas determined while taking into consideration the fact that the g-valuewas close to 2.000.

As described above, it was found that a thin film in which the carbonnanofibers, which are generally dispersed to only a small extent, areuniformly dispersed in the elastomer is obtained according to theinvention. It was found that the thin film exhibits an electricalconductivity similar to that of a metal due to uniformly dispersedcarbon nanofibers.

Examples 11 to 19 and Comparative Examples 4 and 5 (5) Preparation ofSample

(a) Preparation of Carbon Fiber Composite Material

Carbon fiber composite materials of Examples 11 to 19 were obtained inthe same manner as in Examples 1 to 10. In Comparative Example 4, acarbon fiber composite material was obtained by using HAF-grade carbonblack having an average particle diameter of 27 nm (“HAF-CB” in Table 4)in the same manner as in Comparative Example 1. In Comparative Example5, the carbon nanofibers were added and mixed so that the carbonnanofiber content was 40 vol %. However, a carbon fiber compositematerial could not be obtained. The types and the amounts of theelastomer and the carbon nanofibers are shown in Table 4. In Table 4,“DWNT” indicates double-wall carbon nanotubes having an average diameterof 2 nm and an average length of 5 μm. The average lengths of “MWNT” and“SWNT” having the same average diameter as in Examples 1 to 10 were 20μm and 5 μm, respectively.

(b) Preparation of Electron Emission Material

The carbon fiber composite material obtained in each of Examples 11 to18 and Comparative Example 4 was rolled by using rolls and press-formedto prepare a sheet-shaped electron emission material sample having athickness of 1 mm. The electron emission material sample was attached toa cathode substrate formed of copper. An electron emission materialsample of Example 19 was obtained as follows. The carbon fiber compositematerial was supplied to a five-fold amount of toluene and dissolvedwith stirring to obtain a coating liquid. The coating liquid was appliedto a copper substrate by using a screen printing method and then driedto form a thin film with a thickness of 0.05 mm on the substrate. Theelectron emission material samples other than the electron emissionmaterial sample of Example 14 were uncrosslinked. In the preparation ofthe electron emission material of Example 14, 2 phr of peroxide wasadded in the mixing step in (a), and the carbon fiber composite materialwas press-crosslinked at 175° C. for 20 min.

(6) Measurement of Properties

The tensile strength and the dynamic storage modulus (E′) of theelectron emission materials of Examples 11 to 19 and Comparative Example4 were measured. The results are shown in Table 4.

(7) Measurement of Threshold Electric Field and Saturation CurrentDensity

The threshold value and the saturation current density of the electronemission materials of Examples 11 to 19 and Comparative Example 4 weremeasured by using a device as shown in FIG. 9. In the measurement of thethreshold value, voltage was gradually applied between the anode and thecathode, and the electric field (voltage/electrode-to-electrodedistance) at which electron emission started to occur was taken as thethreshold electric field. In the measurement of the saturation currentdensity, voltage was gradually applied between the anode and thecathode, and the value at which the current density was almost saturatedwas taken as the saturation current density. The measurement results areshown in Table 4.

TABLE 4 Example 11 12 13 14 15 16 Carbon fiber Elastomer NR NR NR NR NRNR composite material CNT MWNT MWNT MWNT MWNT DWNT SWNT Amount Elastomer99.5 90 70 90 90 90 (vol %) CNT 0.5 10 30 10 10 10 HAF-CB 0 0 0 0 0 0Electron emission Shape Sheet Sheet Sheet Sheet Sheet Sheet materialAverage thickness (mm) 1 1 1 1 1 1 Crosslinking Uncross- Uncross-Uncross- Cross- Uncross- Uncross- linked linked linked linked linkedlinked Properties Tensile strength 5 13 23 15 12 11 (MPa) E′ (MPa) 12 68760 66 72 69 Cathode substrate Cu Cu Cu Cu Cu Cu Threshold electricfield 3.9 3.3 3.6 3.4 2.1 2.9 (V/μm) Saturation current density 110 420530 400 880 750 of electron emission (mA/cm²) Example ComparativeExample 17 18 19 4 5 Carbon fiber Elastomer EPDM E-SBS NR NR NRcomposite material CNT MWNT MWNT MWNT None MWNT Amount Elastomer 90 9090 90 60 (vol %) CNT 10 10 10 0 40 HAF-CB 0 0 0 10 0 Electron emissionShape Sheet Sheet Thin film Sheet — material Average thickness (mm) 1 10.05 1 — Crosslinking Uncross- Uncross- Uncross- Uncross- — linkedlinked linked linked Properties Tensile strength 7 24 12 9 — (MPa) E′(MPa) 26 160 61 3 — Cathode substrate Cu Cu Cu Cu — Threshold electricfield 3.8 3.1 3.2 — — (V/μm) Saturation current density 360 520 570 — —of electron emission (mA/cm²)

As is clear from Table 4, the following items were confirmed accordingto Examples 11 to 19 of the invention. Specifically, it was confirmedfrom Examples 11 to 19 of the invention that a threshold electric fieldas low as 2.1 to 3.9 (V/μm) is obtained even if the surface of theelectron emission material is not processed at all. It was found thatthe electron emission materials of Examples 11 to 19 had excellentelectron emission properties due to high saturation current density. Itwas also found that the threshold electric field and the saturationcurrent density of the electron emission material are not affected bythe presence or absence of crosslinking and are affected to only a smallextent by the shape of the electron emission material such as a thinfilm or a sheet. Note that electron emission did not occur when usingthe sample of Comparative Example 4.

Although only some embodiments of the invention have been described indetail above, those skilled in the art will readily appreciate that manymodifications are possible in the embodiments without departing from thenovel teachings and advantages of this invention. Accordingly, all suchmodifications are intended to be included within the scope of thisinvention.

1. A substrate having a thin film obtained by: mixing carbon nanofibersinto an elastomer including an unsaturated bond or a group havingaffinity to the carbon nanofibers, and dispersing the carbon nanofibersby applying a shear force to obtain a carbon fiber composite material;mixing the carbon fiber composite material and a solvent to obtain acoating liquid; and applying the coating liquid to a substrate to formthe thin film, the thin film in an uncrosslinked form having a firstspin-spin relaxation time (T2 n) measured at 110° C. of 100 to 3,000μsec, a second spin-spin relaxation time (T2 nn) measured at 110° C. ofeither 0 μsec or 1,000 to 10,000 μsec, a fraction (fn) of componentshaving the first spin-spin relaxation time of 0.95 or more.
 2. Thesubstrate as defined in claim 1, wherein the thin film has a g-value ofa signal of an unpaired electron f carbon measured at 4,5° K by using anelectron spin resonance spectrometer of 2.000 or more and less than2.002.
 3. The substrate as defined in claim 2, wherein the thin film hasa line width of a signal of an unpaired electron of carbon measured at4.5° K by using an electron spin resonance spectrometer of 300 μT ormore.
 4. The substrate as defined in claim 1, wherein the elastomer hasa molecular weight of 5,000 to 5,000,000.
 5. The substrate as defined inclaim 1, wherein at least one of a main chain, a side chain, and aterminal chain of the elastomer includes at least one of a double bond,a triple bond, an α-hydrogen, a carbonyl group, a carboxyl group, ahydroxyl group, an amino group, a nitrile group, a ketone group, anamide group, an epoxy group, an ester group, a vinyl group, a halogengroup, a urethane group, a biuret group, an allophanate group, and aurea group.
 6. The substrate as defined in claim 1, wherein a networkcomponent of the elastomer in an uncrosslinked form has a spin-spinrelaxation time (T2 n) measured at 30° C. by a Hahn-echo method using apulsed nuclear magnetic resonance (NMR) technique of 100 to 3,000 μsec.7. The substrate as defined in claim 1, wherein a network component ofthe elastomer in an crosslinked form has a spin-spin relaxation time (T2n) measured at 30° C. by a Hahn-echo method using a pulsed nuclearmagnetic resonance (NMR) technique of 100 to 2,000 μsec.
 8. Thesubstrate as defined in claim 1, wherein the carbon nanofibers have anaverage diameter of 0.5 to 500 nm.
 9. The substrate as defined in claim1, wherein obtaining the carbon fiber composite material includes tightmilling a mixture of the carbon nanofibers and the elastomer by using anopen-roll method with a roll interval of 0.5 mm or less.
 10. Thesubstrate as defined in claim 9, wherein two rolls used in the open-rollmethod have a surface velocity ratio of 1.05 to 3.00.
 11. The substrateas defined in claim 1, wherein obtaining the carbon fiber compositematerial is performed by an internal mixing method.
 12. The substrate asdefined in claim 1, wherein obtaining the carbon fiber compositematerial is performed by a multi-screw extrusion kneading method. 13.The substrate as defined in claim 1, wherein obtaining the carbon fibercomposite material is performed at 0 to 50° C.
 14. The substrate asdefined in claim 1, wherein forming the thin film is performed by amethod selected from a spin coating method, a dipping method, a screenprinting method, a spraying method, and an inkjet method.
 15. Anelectron emission material comprising a thin film obtained by: mixingcarbon nanofibers into an elastomer including an unsaturated bond or agroup having affinity to the carbon nanofibers, and dispersing thecarbon nanofibers by applying a shear force to obtain a carbon fibercomposite material; mixing the carbon fiber composite material and asolvent to obtain a coating liquid; and applying the coating liquid to asubstrate to form the thin film, the thin film in an uncrosslinked formhaving a first spin-spin relaxation time (T2 n) measured at 110° C. of100 to 3,000 μsec, a second spin-spin relaxation time (T2 nn) measuredat 110° C. of either 0 μsec or 1,000 to 10,000 μsec, a fraction (fn) ofcomponents having the first spin-spin relaxation time of 0.95 or more.16. An electron emission material comprising a carbon fiber compositematerial including an elastomer and carbon nanofibers dispersed in theelastomer, the carbon fiber composite material in an uncrosslinked formhaving a first spin-spin relaxation time (T2 n) measured at 110° C. of100 to 3,0001 μsec, a second spin-spin relaxation time (T2 nn) measuredat 110° C. of either 0 μsec or 1,000 to 10,000 μsec, a fraction (fn) ofcomponents having the first spin-spin relaxation time of 0.95 or more.17. An electron emission device comprising: a cathode including theelectron emission material as defined in claim 15; and an anode disposedat a specific interval from the cathode, wherein the device isconfigured to emit electrons from the electron emission material whenvoltage is applied between the anode and the cathode.
 18. An electronemission device comprising: a cathode including the electron emissionmaterial as defined in claim 16; and an anode disposed at a specificinterval from the cathode, wherein the device is configured to emitelectrons from the electron emission material when voltage is appliedbetween the anode and the cathode.